The telescope was given approval by the state Board of Land and Natural Resources in April 2013,[7] as well as the Intermediate Court of Appeals of the State of Hawai'i.[8] Construction of the telescope began on July 28, 2014[9] and the dedication and ground-breaking ceremony was held on October 7, 2014.[10]

TMT would be a general purpose observatory capable of investigating a broad range of astrophysical problems. A science case prepared by the TMT Foundation outlines the following aims for the observatory:[12]

The TMT began in the 1990s as the California Extremely Large Telescope, and due to a positive response continued to be developed.[13] The project changed its name to Thirty Meter Telescope in 2003-4 as the scope of development partners evolved, including a merger with the Canadian VLOT project and the GSMT.

A description of the TMT Observatory design can be found in the TMT Construction Proposal (2007).[14]

The centerpiece of the TMT Observatory is to be a Ritchey-Chrétien telescope with a 30-metre (98 ft) diameter primary mirror. This mirror is to be segmented and consist of 492 smaller (1.4 m), individual hexagonal mirrors. The shape of each segment, as well as its position relative to neighboring segments, will be controlled actively.[citation needed]

A 3-metre (9.8 ft) secondary mirror is to produce an unobstructed field-of-view of 20 arcminutes in diameter with a focal ratio of 15. A flat tertiary mirror is to direct the light path to science instruments mounted on large Nasmyth platforms.[citation needed]

The telescope is to have an altitude-azimuth mount. This mount will be capable of repositioning the telescope between any two points of the sky in less than 5 minutes, with a precision of 2.0 arcseconds or better. Once the celestial object is acquired, the telescope will track its motion with a precision of a few milliarcseconds.[citation needed]

Integral to the observatory is a Multi-Conjugate Adaptive Optics (MCAO) system. This MCAO system will measure atmospheric turbulence by observing a combination of natural (real) stars and artificial laser guide stars. Based on these measurements, a pair of deformable mirrors will be adjusted many times per second to correct optical wavefront distortions caused by the intervening turbulence.

Three instruments are planned to be available for scientific observations:

Wide Field Optical Spectrometer (WFOS) providing near-ultraviolet and optical (0.3–1.0 μm wavelength) imaging and spectroscopy over a more than 40 square arcminute field-of-view. Using precision cut focal plane masks, WFOS would enable long-slit observations of single objects as well as short-slit observations of hundreds of objects simultaneously. WFOS would use natural (uncorrected) seeing images.

For planning purposes, TMT has developed concepts for an additional six instruments, which it proposes to be deployed during the first decade of science operations. These plans have been reviewed and updated on a roughly bi-annual basis starting in 2010.

In no order of preference, planned additional scientific capabilities include:

The TMT Observatory Corporation board of directors narrowed the list to two sites, one in each hemisphere, for further consideration: Cerro Armazones in Chile's Atacama Desert, and Mauna Kea on Hawai'i Island. On July 21, 2009 the TMT Board selected Mauna Kea as the preferred site.[16][17] The final TMT site selection decision was based on a combination of scientific, financial, and political criteria; ESO is also building a very large telescope E-ELT, and is doing so in Chile. If both next-generation telescopes were in the same hemisphere, there would be many astronomical objects that neither could observe.

The telescope was given approval by the state Board of Land and Natural Resources in April 2013.[7] However, there was some opposition in Hawaii to the building of the telescope,[18] based on potential disruption to the fragile glacial environment of Mauna Kea due to construction, traffic and noise, which is a concern for habitat disruption of several species,[19] and to the fact that Mauna Kea is a sacred site for the Native Hawaiian culture.[20][21] Hawaiian cultural practitioners cite impacts to indigenouscultural practice, while recreational users have argued that construction harms the scenic viewplane, and environmentalists are concerned that irreparable ecological damage may be done by construction. All three groups are represented amongst the petitioners opposing the TMT.[22] According to State of Hawaiʻi law HAR 13-5-30, eight key criteria must be met before construction can be allowed on conservation lands in Hawaiʻi. Among other criteria, the development may not “cause substantial adverse impact to existing natural resources within the surrounding area, community, or region,” and the "existing physical and environmental aspects of the land must be preserved or improved upon.[23] "

The Hawaii Board of Land and Natural Resources conditionally approved the Mauna Kea site for the TMT in February 2011. The approval has been challenged; however, the Board officially approved the site following a hearing on February 12, 2013, and the TMT Foundation anticipates that construction will begin in April 2014.[24]

The current US$80 million, five-year design and development program is planned for completion in 2012.[27] Construction is expected to commence immediately thereafter, leading to initial science operations in 2018.[27] The Gordon and Betty Moore Foundation has committed US$200 million for construction. Caltech and University of California have committed an additional US$50 million each. TMT is actively seeking additional major partners for the construction and operations phase.

In 2008, the National Astronomical Observatory of Japan (NAOJ) joined TMT as a Collaborating Institution.[28]

In 2009, the National Astronomical Observatories of the Chinese Academy of Sciences (NAOC) joined TMT as an Observer.[29][30]

In 2010, a consortium of Indian Astronomy Research Institutes (IIA, IUCAA and ARIES) joined TMT project as an observer. The observer status is the first step in becoming a full partner in TMT and participating in the engineering development and scientific use of the observatory (Subject to approval of funding from Indian Government).

In 2012, India and China became partners, with representatives on the TMT board. China and India will pay a share of the telescope construction costs, expected to top $1 billion.[31][32]

Japan, which has its own large telescope at Mauna Kea, the 8.3-metre Subaru, is also a partner.[33]

TMT has received design and development funding from the following public and private organizations:

The telescope cost was estimated in 2009 to be $970 million[34] to $1.4 billion;[16] the funding had not been completely raised by mid-2011, although $100 million had already been spent on design, engineering and site-assessment work.

Comparison of nominal sizes of primary mirrors of the Thirty Meter Telescope and some notable optical telescopes (click for detail)

Hubble floats free after its 2009 servicing

At wavelengths longer than 0.8 μm, adaptive optics correction would enable observations with ten times the spatial resolution of the Hubble Space Telescope. TMT would be more sensitive than existing ground-based telescopes by factors of 10 (natural seeing mode) to 100 (adaptive optics mode). If completed on schedule, TMT could be the first of the new generation of Extremely Large Telescopes.